The 100,000 Year Problem

Scientists who study climate will tell you that today's warm temperatures and mild conditions are not normal for Earth during the past several million years. Our planet has been in a general cooling trend for 35 million years and in the grip of an Ice Age for the last 1.6 million years. What's more, this Ice Age, known as the Pleistocene, consists of relatively short periods of warmth, called interglacials, separated by much longer periods of bitter cold, referred to as glacials. The recorded history of humankind covers only the later half of the most recent interglacial warming, though our ancient ancestors did leave messages in the form of cave art that date back to much colder times, times when the Ice Age held the world fast in its frozen embrace. Predicting the timing and duration of these periods remains a problem for scientists. Why the glacial periods should last around 100,000 years, as they have for the last million years or so, is called the 100-kyr problem. Now, a group of researchers claim they know the answer.

There are many cycles buried in the record of Earth's fluctuating climate. Most intriguing among them has got to be the waxing and waning of glacial conditions, which have covered large areas of the northern hemisphere under kilometers of ice in the past. Our warm period, the Holocene, started about 11,000 years ago, which means that it may be about to end. Most scientists believe that the Pleistocene Ice Age is not over and that our world will return to glacial conditions in the near future. Being able to predict this climate oscillation would be useful, to say the least.

In the past, climate cycles have been linked to changes in Earth's orbital parameters, which change in a quasi-periodic way. These changes are called the Milankovitch Cycles, after Serbian astrophysicist Milutin Milankovitch who dedicated his life to developing a mathematical theory of climate. He was the first to systematically analyze the connection between Earth's orbit and climate change.

The Milankovitch Cycles: Precession, tilt and eccentricity.

But with nature, nothing is ever simple. It turns out that the glacial-interglacial cycle used to take only 41,000 years. Then, around 1.2 million years ago, this shifted to the current 100kyr cycle. Scientists have found the “fingerprint” of the Milankovitch Cycles in the proxy record but the case for the longer, 100,000 year cycle is not as accepted as the 41,000 year cycle. Indeed, the duration of each of the last four glacial cycles increased from 80 to 130 thousand years, which suggests that these major climate shifts are aperiodic. Many think that a clearer explanation is needed.

Recently, José A. Rial, Jeseung Oh and Elizabeth Reischmann published a letter in Nature Geoscience in which they claim that forced synchronization can explain the strong 100,000-year glacial cycles through the alignment of insolation changes and internal climate oscillations. Here is how they describe their study, “Synchronization of the climate system to eccentricity forcing and the 100,000-year problem”:

Synchronization is a fundamental nonlinear phenomenon and one basic mechanism of self-organization in complex systems, and synchronization of nonlinear oscillators to external forcing (see Supplementary Information) is commonly encountered in physics, chemistry, biology, engineering and climatology. We shall use the term forced synchronization (also called master–slave) to describe how the climate system (slave) gradually adjusted its natural rhythms to those of the forcing (master) over the past 5 million years. We will discuss evidence suggesting that forced synchronization by the 413-kyr orbital eccentricity (which modulates the amplitudes of the orbital 100-kyr eccentricity and precession) phase locked, frequency entrained, frequency modulated, and amplified the free oscillations of the climate system during the past 1.2 Myr. The evidence for this comes mostly from analyses of available data, particularly the 5-Myr-long LR04 stack of 57 globally distributed δ18O benthic proxies of total ice volume extensively reported on elsewhere. In addition, we assume that the climate system on the timescale of interest can be modelled as a nonlinear, externally forced self-sustained oscillator with free periods in the range of 104–105 years. This model will be used to help interpret key features of the LR04 stack and to demonstrate an example of how eccentricity synchronization may occur.

The authors first compared the LR04 time series to a simple modulated harmonic signal, “We find compelling evidence that the climate system’s ~ 100-kyr glaciation cycles are frequency modulated by the 413-kyr component of eccentricity,” they report. To understand how a 413 kyr cycle can induce a 100 kyr climate cycle consider the fact that the cycles under discussion are not simple sine waves. They are complex composites with many harmonic frequencies contained within their signals. This is shown in the figures below.

Rial et al. claim that the presence of this modulation in the evolution of the Ice Age climate implies the existence of a modulator, a time function that can reveal key aspects of the response of the climate system to the forcing. “Known in electronic parlance as the intelligence of the modulated signal, the modulator must exist throughout the entire length of the LR04 record, because the astronomical forcing is always present,” they state. Fortunately, there are simple and well-tested mathematical techniques for extracting a modulator from a modulated time series.

“We chose the most direct method applicable to the available data, by which the modulator function is obtained from the envelope of the rectified time derivative of the low-pass filtered untuned 5-million-year-long LR04 stack,” the letter explains. This extracted modulator, shown in the figure below, is the orbital forcing function experienced by the climate system. The authors describe it as “a quasi-periodic function with power at frequencies in the 1/800–1/300-kyr bands, and a proxy for the climatic response.”

The filtered trace, shown above, is the average of singular spectral analysis (SSA) and empirical mode decomposition (EMD). For a detailed explanation of how this forcing drives climate variation to synchronize with the ~100kyr harmonic, see the paper. The bottom line is: “This theoretical result shows that if the natural frequencies of oscillation of the climate system are within 10–20% of the forcing period, synchronization occurs even for very small forcing amplitude and can produce a power gain (ratio of the synchronized power to the forcing power) of 300–700% and greater for small detuning.” The paper's conclusions are as follows.

We conclude that the inconsistencies discussed in the introduction can be explained as caused by forced synchronization. Synchronization allowed energy from the sun to flow into or out of the climate system at the same time internal feedbacks were warming or cooling it, resulting in unprecedentedly large climate fluctuations that powered the great Pleistocene glaciations (a process akin to resonance of a forced linear oscillator). Forced phase synchronization, which is still occurring, started 1.2 Myr ago, and culminated at the time of the MIS11 (~ 0.4 Myr) with a brief period of nonlinear resonance... Today resonance has faded but frequency modulation persists, driving the ~ 1/82–1/125-kyr frequency deviation that paces the timing of the major glacial terminations.

The conclusions we should draw from this is that orbital variations are the primary driver of climate change on Earth. Over time the effective period of the forcing may change as resonance frequencies shift but it is a combination of planetary motion and solar insolation that moves our climate into and out of glacial periods. All evidence indicates that the 100kyr cycle persists and our planet is headed for another period of glaciation, a period that will cause greater problems for human civilization than any of the predicted ravages of anthropogenic global warming. Unlike global warming this threat is for real. In short, do not fear the heat, fear the cold.

Be safe, enjoy the interglacial and stay skeptical.

The future looks much colder, so enjoy the interglacial!